1. Field
Example embodiments relate to a switching device of an active display device and methods of driving the switching device, and more particularly, to a switching device of an active display device and a method of driving the switching device, the switching device having an improved electrical reliability.
2. Description
An active display device includes a switching device that controls an operation of each pixel of the active display device. A thin film transistor (TFT) is used as a switching device for a display. For example, one pixel of the display includes one TFT as a switching device. The TFT may be, for example, a silicon-TFT, an oxide TFT, an organic TFT, or the like according to type of a semiconductor material used as a channel material. An oxide TFT having rapid switching speed is used as the switching device.
The switching device allows a desired voltage to charge a pixel unit by making a current flow in a channel of the TFT for a desired period of time. After the desired period of time, the switching device maintains the pixel unit in a charged state by turning off the TFT. In an active matrix liquid crystal display (AMLCD), a period of time in which the TFT is ON is determined according to a driving frequency and a resolution of the AMLCD. In the case of a driving operation at 120 Hz with a full high-definition (HD) class, a period of time in which one TFT is an on state is about 1/120/1080=7.7 μs. Then, for a remainder of one period (1/120=8.3 ms), the TFT remains in an OFF state. Thus, with respect to one period, the TFT of the active display device is mostly in an OFF state.
An amorphous silicon TFT or an oxide semiconductor TFT exhibits an n-type semiconductor characteristic, and thus, in order to turn off such a TFT, a negative gate voltage is applied to the TFT. Thus, the negative gate voltage is continuously applied to the TFT while the TFT is in the OFF state in the active display device. However, when the negative gate voltage is continuously applied to the TFT for a desired time period, a threshold voltage may shift toward a negative voltage. As a result, a leakage current may increase as the negative gate voltage is applied to the TFT. The shift of the threshold voltage may be severe when light is incident on the switching device. When the leakage current increases, image quality of the active display device may deteriorate.